This invention relates to novel epoxy adhesive compositions, to fiber-reinforced composites incorporating such adhesives, and to cured composites.
Epoxy resins have been known for several decades and are widely used industrially. These resins are readily crosslinked, or cured, with a wide variety of "hardeners," accelerators often being added to increase the rate of reaction. When reinforced with fibers, especially glass or graphite fibers, these resins form lightweight construction materials that are often employed in the manufacture of sports equipment or in the aeronautic and aerospace industries. In preparing fiber-reinforced products of this type, it is common to saturate a sheet, web, or fabric of fibers (e.g., nonwoven, stitch-bonded, knit, woven, or aligned filaments) with a viscous epoxy resin containing a suitable curing system, thereby forming what is known as a "prepreg," which is subsequently shaped to the desired contours and heated to cure the resin.
A common use for prepregs is in the repair of thermoset or thermoplastic laminates, including failed bonds between thermoset and thermoplastic surfaces. One particularly demanding use is in the repair of aircraft structures, e.g., those in which a reinforcing and supporting "honeycomb" core is covered with a thin skin; cf., e.g., U.S. Pat. No. 4,311,661. Since such repairs must often be carried out on contaminated surfaces in inhospitable environments that may be hot, humid, cold, or dusty, field personnel have listed a number of features that prepregs should possess. For example, the resin should be completely curable within two hours and, since the damaged substrate may contain moisture, the curing temperature should be below 100.degree. C., reducing the possibility of generating steam that might weaken, or even destroy, the repaired section. Because refrigerated storage facilities may not be available, it is important for the prepreg to have a shelf life of at least a year at room temperature (20.degree.-25.degree. C.). Prior to the present invention, no epoxy resin composition has been able to meet these criteria.
U.S. Pat. No. 4,594,291 discloses a curable epoxy resin composition employing, among other things, a sulfanilamide curing agent, and a tetrabutyl phosphonium acetate-acetic acid catalyst. This composition which is cured at temperatures above 140.degree. C., has inadequate room temperature shelf life and lacks toughness.
Japanese Patent Publication No. 57-018551 discloses a curable epoxy adhesive comprised of a poly(para-hydroxystyrene) extender polymer and an undisclosed cure accelerator. The cured adhesive exhibits improved heat, alkali, and chemcial resistance but does not cure at low temperature and has inadequate shelf life.
Japanese Patent Publication No. 60-028425 discloses an epoxy resin composition containing a para-tertiarybutyl phenol polymer extender, dicyandiamide as a hardener, and dichlorophenyl-1,1 dimethyl urea as a cure accelerator. This system is readily workable and cures at low temperature but lacks sufficient shelf life. The cured resin is not sufficiently resistant to moisture, especially at elevated temperatures.
U.S. Pat. No. 4,659,779 discloses solid solutions of imidazole cure accelerators dissolved in poly(para-hydroxystyrene) polymer that are capable of producing curable epoxy adhesives with adequate shelf lives. None of the cured epoxy adhesives in the examples possesses the desired properties previously specified.
European Patent Publication No. 165,230 discloses an epoxy adhesive that uses boron trifluoride complex as a catalyst. This composition is easy to work with and cures at low temperatures but lacks adequate shelf life. The cured resin also lacks the desired properties previously specified.
U.S. Pat. Nos. 3,519,576 and 3,520,905 describe latent curing agents for epoxy resins comprised of amine phenolate salts that provide adequate shelf life and cure at temperatures below 100.degree. C. but the cured resin compositions lack the requisite physical properties.
U.S. Pat. No. 3,884,992 discloses curable epoxy compositions employing poly(para-hydroxystyrene) polymer as an extender. The cured compositions have good heat resistance but are deficient in the other desired physical and mechanical properties.